This invention relates to laser beam generators, and particularly to one in which a wavelength-converted laser beam is generated by a nonlinear optical crystal element.
A laser beam generator is proposed in which a nonlinear optical element is disposed within a resonator so that the wavelength conversion by the nonlinear optical element is efficiently made by use of a high-power density within the resonator.
This type of laser beam generator proposed includes, for example, an SHG (second harmonic generator) of external resonator type, or an SHG having a nonlinear optical element provided within a laser resonator.
The SHG of external resonator type has a nonlinear optical element provided between a pair of opposite mirrors constituting the external resonator. A fundamental laser beam is incident to this external resonator, and passed through the nonlinear optical element. The external resonator in this case has a length selected for the resonance to the frequency (wavelength) of the incident laser beam.
In this SHG, of external resonator type, for example, the so-called finesse value (corresponding to Q in resonance) of the resonator is selected to be as high as, for example, about 100 through 1000, so that the light density within the resonator is increased several hundred times the incident light density, thereby making it possible that the nonlinear optical crystal element is effectively operated within the resonator.
In addition, a light source for the incident laser beam to, for example, this external resonator type SHG is formed of a laser medium and, for example, a nonlinear optical element which are disposed between a pair of opposite mirrors constituting a laser resonator. The laser beam emitted as the fundamental wave from the laser medium by, for example, irradiating an exciting light beam on the medium is converted into an SHG laser beam by the nonlinear optical element within the laser resonator. This laser beam is made incident to the external resonator in which the nonlinear optical element is disposed as described above.
In the laser beam generator in which a second harmonic laser beam or higher-order harmonic or sum-frequency laser beam is converted in wavelength by the external resonator, the change (error) of light path length of the external resonator must be controlled to be limited to an extremely small range of 1/1000 through 1/10000 of the resonant wavelength, or under 1.ANG., that is, an extremely high- precision positional control is necessary.
Thus, for example, the mirrors constituting the external resonator are supported by a laminated piezoelectric element so as to move a very small distance for adjustment in the optical axis direction, and the error signal proportional to the deviation of the resonator length relative to the incident laser beam to the external resonator is fed back to the laminated piezoelectric element, or a servo loop is formed, so that the resonator is automatically controlled in its length, thereby stabilized in its resonance operation to the laser beam which is incident to the external resonator.